Mature cystic teratomas (also called dermoid cysts) account for about 30–45 % of all ovarian neoplasms and around 60 % of all benign tumours arising in the ovary.1 Malignant transformation of the various mature tissue components of a dermoid cyst is rare and the reported incidence is 0.17–1.4 %.2,3 Squamous cell carcinoma (SCC), arising from the ectodermal component, is the commonest form of malignant transformation accounting for >80 % of cases, followed by adenocarcinomas and carcinoid tumours.4 SCC in mature cystic teratomas is most commonly seen in postmenopausal women.4 There are no distinctive clinical features, tumour markers are often normal and pre-operative radiological diagnosis is difficult. Malignant transformation of an ovarian cyst is therefore difficult to predict, hence most cases are diagnosed postoperatively. 5 Tumours confined to the ovary usually have a better prognosis and fertility-sparing surgery is an option where appropriate.6 Conversely, patients with stage III or IV disease rarely survive five years.7 Our knowledge about these rare tumours is limited and based mainly on small case series and case reports and the optimal management of this condition is unclear. Whilst surgery is often performed initially, the appropriate adjuvant treatment following surgery is also unclear. The aim of this study is to present the experience of our centre in managing this rare tumour and review the key literature in this area. We hope this contributes to the evidence base and helps inform on the most appropriate management strategy for this rare type of tumour. Patients and Methods During a 24-year period (1986–2010) we retrospectively identified six patients treated for SCC arising in a mature cystic teratoma of the ovary at Velindre Cancer Centre, UK. We collected demographic and clinical outcome data from patient records and overall survival was defined from the date of diagnosis to the date of death. For each case we recorded patient characteristics, presenting symptoms, pre-operative tumour markers and imaging, tumour stage, treatment and outcome. Results The age range of patients at diagnosis was 42–65 years with a median age of 56 years (see Table 1). Five women presented with chronic abdominal/ pelvic pain whilst one presented with acute abdominal pain and vomiting. Four of the patients had ultrasound scans, two had CT scans and one had a MRI scan as their pre-operative imaging study. Only one patient in our series, upon CT scan, was suspected pre-operatively to have malignant transformation in a dermoid cyst. The other patients had a diagnosis of a complex ovarian cyst. Pre-operative CA-125 was measured in all patients and was elevated in four patients and normal in two. Four patients had imaging studies available for review and the maximum tumour dimension range was 10–15 cm with a median of 10 cm. All six women underwent staging laparotomy and optimal cyto-reduction was achieved in only one patient (see Table 2). Two women had sub-total hysterectomies, one had a small bowel resection whilst another woman required a colostomy. None had lymphadenectomy as part of their surgical staging. One woman had stage IIC disease and was optimally debulked whilst the remaining patients all underwent surgical debulking but none achieved optimal cytoreduction. Of the patients who didn’t achieve optimal cytoreduction three had stage IIIc disease while the other two had stage IV disease. All patients had adjuvant carboplatin based chemotherapy and three women had paclitaxel and one had docetaxel additionally, as part of first-line chemotherapy. An example of a haematoxylin and eosin microscopic histology section from case 6 of an area of well-differentiated SCC arising in a dermoid cyst is shown in Figure 1. Two women achieved partial response following six cycles of chemotherapy. One had second line chemotherapy with pegylated liposomal doxorubicin for disease progression three months after completing first-line chemotherapy and died from disease progression 13 months from initial diagnosis. The second patient who responded had palliative radiotherapy to the lumbar spine for symptomatic management of progressive skeletal metastasis and survived 17 months from diagnosis. Of the other patients, one woman died after the fourth cycle of carboplatin chemotherapy from progressive disease, six months from initial diagnosis. One woman had stable pelvic disease after six cycles of combination chemotherapy and had a course of pelvic radiotherapy (45 Gy/25#). She had disease progression while on radiotherapy and survived a total of 12 months from diagnosis. The other two women had progressive disease on combination chemotherapy. One was switched to palliative pelvic radiation (25 Gy/10#) after five cycles of chemotherapy and died while on treatment, seven months from diagnosis. The other woman had chemo-radiation (cisplatin + 5-flurouracil with pelvic radiation 45 Gy/25#) for localised pelvic disease following six cycles of combination chemotherapy and had a partial response. She survived for 19 months, the longest in our series, from her initial diagnosis (see Table 2). Discussion Mature cystic teratoma of the ovary, often referred to as dermoid cysts, are benign and are one of the commonest ovarian tumours. They occur principally in premenopausal women with a mean age at diagnosis of 35 years.3 Malignant transformation of these tumours is rare and occurs in 0.17–1.4 % of cases.2,3 This usually occurs in postmenopausal women with studies showing a mean age of onset of 55 years,4 which compares well with the mean age of diagnosis in our patients. The average diameter of benign dermoid cysts is 6–7 cm2,3 while that of the malignant counterpart is 14 cm.4 The mean tumour diameter in our series is around 10 cm.